1. Field of the Invention
This invention relates to a fire retardant polycarbonate resin composition, and more particularly, to a fire retardant polycarbonate resin composition containing diantimony tetroxide.
2. Description of the Prior Art
Since polycarbonate resins have excellent thermal, electrical and physical properties, they are widely used in various fields. However, as the result of the recent strict regulation relative to physical properties of resins, even a polycarbonate resin known as a resin having well balanced excellent properties requires improvement in some properties. A typical property thereof is combustibility and UL 94 "Tests For Flammability of Plastic Materials For Parts In Devices And Appliances" (Underwriters Laboratories, Inc.) (hereinafter referred to as UL 94) is applied to resins as an authoritative standard for combustibility. According to UL 94, polycarbonate resins are self-extinguishing, but during combustion, the flaming particles drip, and they are classified in Self-extinguishing Group V-II depending upon the thickness of the test piece. Such combustibility is not desirable from a commercial point of view. Therefore, various efforts have been made to improve the fire retardancy.
The methods for improving the fire retardancy generally involve suppressing the combustibility by introducing an appropriate substituent into the polycarbonate molecule or adding a compound capable of suppressing the combustibility of the polycarbonate.
An example of the former method is disclosed in Japanese Patent Publication No. 24660/1972. The method comprises producing a polycarbonate containing, as monomeric units, 2,2-bis(4-hydroxy-3,5-dibromophenyl) propane and 2,2-bis(4-hydroxyphenyl) propane, and features using bromine as the substituent. However, when the content of 2,2-bis(4-hydroxy-3,5-dibromophenyl) propane monomer units increases, the moldability is disadvantageously lowered, though the fire retardancy increases. Moreover, the molding temperature is so high that bromine and/or a bromine compound is liberated and the resulting shaped article suffers from foaming and has a poor appearance resulting in lowering the commercial value.
Therefore, this method improves the fire retardancy at the expense of the excellent properties inherent to polycarbonate resins. It is not a desirable method.
An example of the latter method is incorporating a halogen compound into the polycarbonate resin. Japanese Patent Publication No. 44537/1972 discloses that a carbonate oligomer (average degree of polymerization being 2-10) derived from 2,2-bis(4-hydroxy-3,5-dibromophenyl) propane is added to a polycarbonate resin as the halogen compound taking the compatibility with the polycarbonate into consideration. According to this method, the migration of the halogen compound to the surface of the shaped article is less than that of other halogen compounds. However, when the amount of the carbonate oligomer is increased so as to enhance the fire retardancy, the impact strength and rigidity of the shaped article is disadvantageously lowered to a great extent. And this method is not desirable either because the improvement of fire retardancy is effected to the detriment of other properties of polycarbonate resins. Addition of diantimony trioxide together with the halogenated compound is known as a means for decreasing the amount of the halogenated compound to be added. When this method is applied to polycarbonate resin, the combustion can be retarded, but the thermal stability upon melt molding and physical properties of the molded articles are deteriorated, and further the appearance of the molded articles is markedly affected.
The present inventors have investigated the above mentioned drawbacks and found that diantimony trioxide reacts with the carbonate bond to degrade the polycarbonate and generate carbon dioxide, resulting in deterioration of the appearance of the molded articles. The degradation mechanism seems to be as shown below: ##STR1##
The present inventors have completed the present invention as the result of research in various antimony compounds.